class Solution {//leetcode852——山脉数组的峰顶索引
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left=0,right=arr.size()-1;
        while(left<right){
            int mid=(right-left)/2+left;
            if(arr[mid]>=arr[mid+1]) right=mid;
            else if(arr[mid]<arr[mid+1]) left=mid+1;
        }
        return left;
    }
};